In recent years, the electrophoretic display methods, which employ the electrophoresis of charged fine particles (electret fine particles), have been attracting attention as the most promising technology for next-generation display devices. However, this technology still has many problems, including the shape of charged fine particles, small and unstable charge potential (ζ potential), secondary aggregation or sedimentation of electrophoretic particles, inadequate deletion of previously displayed images, unsatisfactory response speed, and the like. Therefore, research and development has been conducted to correct such shortcomings.
Patent Documents 1 and 2 disclose electret particles for the above applications.
Patent Document 1 discloses negatively charged fine particles produced through addition of a resin serving as an electron trap to a superfine spherical core resin particles having a particle diameter of 1-10 μm obtained by polymerization of a polymeric fine particle material and through irradiation of the particles with a 10-300 kGy electron beam to form negatively charged electret fine particles, wherein the core resin is colored with a desired color (claim 1).
Patent Document 2 discloses colored and negatively charged microparticulates obtained by adding an electron trap material, a pigment, etc., to a polymeric fine particle monomer material; producing spherical fine particles having a particle diameter of 5-10 μm through suspension polymerization, emulsion polymerization, dispersion polymerization or the like; and forming negatively charged electret microparticulates either by irradiating the particles with a 10-50 kGy electron beam and heating the particles for a ten or so minutes at 90 to 110° C., or irradiating the particles with a 10-50 kGy electron beam at 90 to 110° C., wherein the negatively charged electret microparticulates are colored with a desired color and have a ζ potential of −50 to −100 mV (claim 10).
However, the various hitherto-known electret particles are often unevenly electrified and thereby have a problem of insufficient electrophoretic properties. Therefore, development of electret particles that can be uniformly electrified and have excellent electrophoretic properties has been desired. Further, since electret coarse powder is suitable for the electrophoretic particles for large-screen displays, development of electret coarse powder having the above advantageous characteristics has also been desired.